Recently, direct lamination of a thermoplastic resin film to a metal sheet such as electrotinplate, tin free steel (TFS) and aluminum sheet has been attempted instead of lacquer coating, with the intention to prevent environmental pollution caused by discharge of solvent during curing the coated lacquer and decreased energy costs. For example, U.S. Pat. No. 4,517,255 and Japanese Patent Application No. Hei 2-418199 describe such a process.
In particular, U.S. Pat. No. 4,517,255 relates to a process for lamination of a crystalline polyester resin film to a metal sheet heated to a temperature of the melting point of the employed polyester resin and thereafter immediately quenching the laminate. The laminate according to this patent has been used for can stock such as drawn cans, can ends and five gallon cans because the laminated crystalline polyester resin film is sufficiently adhered to the metal sheet by an amorphous non-oriented polyester resin layer which is formed at the interface of the crystalline polyester resin film and the metal sheet as a result of the heating step. This laminate can be formed to a drawn and ironed can and a drawn and stretch formed can wherein more severe formability is required, if almost all of the laminated polyester resin film changes to an amorphous non-oriented state. However, the corrosion resistance in the formed part of the laminate becomes poor by this change of the laminated polyester resin film.
In order to improve the formability of this laminate described above, Japanese Patent Application No. Hei 2-418199 was proposed. In this reference there is described a copolyester resin film laminated metal sheet for a drawn and stretch formed can which comprises a 0.01 to 0.15 planar orientation coefficient in the outermost layer of the laminated polyester resin film and a 0 to 0.10 planar orientation coefficient in the polyester resin layer contacting with the surface of a metal sheet after lamination of copolyester resin film to the metal sheet. Presently, the laminate produced under the restricted conditions of this patent application are used for a drawn and stretch formed can which is treated with hot steam having a temperature of 120.degree. to 130.degree. C. in a retort after packing drinks such as green tea, black tea or coffee drink. This laminate can also be used for a drawn and stretch formed can packed with more corrosive foods and drinks, if the cans are produced and packed under normal conditions, i.e., conditions under which dents do not arise during production, packing or transport. However, some dents may arise in parts of the can body and can bottom by impact with other cans in the can making process or the transporting process after packing foods and drinks. The parts of the inside of the can body and the can bottom by impacts from the outside of the can body can become locally corroded, due to cracks arising in the laminated copolyester resin film.
Therefore, this laminate cannot be used for drawn and stretch formed cans in which severely corrosive foods and drinks are packed, if the cans may be subjected to denting during production, packing or transport.
In order to solve this problem described above, a polyester resin film laminated metal sheet according to Japanese Patent Application No. Hei 2-418198 has been developed. The laminate according to this patent is able to solve the problem described above. However, this laminate cannot be used for a drawn and stretch formed can in which carbonate beverages having severe corrosivity are packed at temperatures below about 5.degree. C., as many cracks arise in the laminated polyester resin film on the inside of the can body by the impact of cans against each other in the transporting process after packing the beverage at low temperature. Laminates produced according to this patent do not have good resistance to denting at low temperatures.
Furthermore, in the production of these laminates, it is necessary that the set temperature in laminating the polyester resin film is controlled within a narrow range, because it is otherwise impossible to obtain a laminate having satisfactory characteristics required for drawn and stretch formed cans.
Recently, it has been proposed that all of these problems can be solved by the formation of a double thermoplastic resin layer consisting of an outer layer of polycarbonate resin and an inner layer of polyester resin on a metal sheet (Japanese Patent Application No. Hei 5-32491). Certainly, the laminate according to this Japanese Patent Application has excellent characteristics which can be applied for a drawn and stretch formed can and other can stock. However, it was recently found that the flavour of foods and drinks packed in the can produced by this laminate changed slightly during storage by adsorption of some composition of foods and drinks to the laminated polycarbonate resin outer layer contacting directly with foods and drinks.